Any hardware store. Trisodium Phosphate, TSP. About $3.00 a pound.
No, I have no idea how much you would have to use, but it wouldn’t be a lot....
I just checked my cupboard and I have three boxes of the highly rated Walmart Great Value Dishwashing Detergent. The almost empty box contains phosphorus. The two smaller boxes contain no phosphates.
The box with phosphates says the phosphorus content is 6%.
You could do the math and know how much tri sodium phosphate to add to a non phosphate product.